Activity and selectivity regulation through varying the size of cobalt active sites in photocatalytic CO2 reduction

Article abstract of DOI:10.1039/c8ta06151c

The development of efficient and economical catalysts for photocatalytic reduction of CO₂ into chemical feedstocks is highly desirable for addressing both the global energy crisis and carbon emission problem. Herein, a series of carbonized coba

Full text of DOI:10.1039/c8ta06151c

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
rsc.li/materials-a

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DOI: 10.1039/C8TA06151C
Journal Name
ARTICLE
Activity and Selectivity Regulation through the Size of Cobalt
Active Sites in Photocatalytic CO Reduction
2
ZIFReceived 00th January 20xx,
Accepted 00th January 20xx
Qiaoqiao Mu , Wei Zhu , Gangbin Yan a,b, Yuebin Lian a,b, Yuanzhou Yao a,b, Qin Li a,b, Yuyu
a,b
a,b
c
c
a, b,*
, Zhao Deng a, b,*
Tian , Peng Zhang , Yang Peng
DOI: 10.1039/x0xx00000x
2
The development of efficient and economical catalysts for photocatalytic reduction of CO into chemical feedstocks is
www.rsc.org/
highly desirable for addressing both the global energy crisis and carbon emission problem. Herein, a series of carbonized
cobalt composites derived from the bimetallic Zn/Co zeolite imidazolate frameworks (C-BMZIFs) are synthesized and used
2+
as the co-catalysts for [Ru(bpy)
ratio allows regulating the size of Co active sites, which further differentiates their catalytic activity towards CO
and H evolution, resulting in tunable CO/H ratio in the produced syngas. Among all investigated configurations, the C-
3
]
mediated photocatalytic CO
2
reduction under visible light irradiation. Varying the Zn/Co
2
reduction
2
2
4
BMZIF with a Zn/Co ratio of 3:1 demonstrates the highest CO yield of 1.1  10 µmol/g/h, owing to the optimal balance
between the quantity of active sites and the activity of each individual site. Consequently, this work provides insight into the
2
design of stable MOFs-derived co-catalysts for efficient photo-reduction of CO , and offers an alternative solution for
photocatalytic syngas production with tunable CO/H ratio.
2
certainly desired because of the simplification of the following
separation process, in some applications, especially for the use of
1
. Introduction
syngas containing CO and H
Tropsch reactions, the HER can be indeed beneficial and utilized for
simultaneous H
production.17, 20, 21 This is even more so if one can
conveniently regulate the selectivity, and thus the CO/H ratio in the
obtained syngas, targeting for the synthesis of specific chemicals.
2
as the chemical feedstock for Fischer-
Artificial photosynthesis aiming for the photocatalytic conversion of
CO into renewable fuels and chemical feedstocks has been regarded
as a promising solution to simultaneously reduce environmental
pollutions and alleviate the growing energy crisis.1-4 A key aspect in
realizing this goal is the development of highly efficient
photocatalysts that enable to greatly lower the kinetic energy barrier
2
in CO reduction, which requires to break the stable C=O bond (750
2
2
2
1
7,
1
8, 20
Metal-organic frameworks (MOFs), capable of integrating CO
2
-
1
capture and its catalytic conversion together, represents a class of
porous materials that have been demonstrated as efficient co-
catalysts, in work with photosensitizers together, for photocatalytic
kJ mol ) and is initiated by the formation of the thermodynamically
•-
unfavorable intermediate CO
2
upon accepting the first electron
•-/CO ) = -1.49 V vs. RHE).4-6 In such context, tremendous
2
E0(CO
(
2
CO
promoting the kinetics of both charge separation from the
photosensitizer and surface reaction for CO
conversion.12, 25 Taking
reduction.3, 10-12, 22-24 They act as the catalytic booster by
2
efforts have been devoted to develop potent and economic catalysts
RR).2, 7-15 Of the diverse
for the CO reduction reaction (CO
reduction products, CO has been mostly obtained, as the conversion
from CO to CO only involves two electron transfer and a redox
potential of -0.12 V vs. RHE.8, 16 However, in many catalytic
2
2
2
the Zeolitic Imidazolate Frameworks (ZIFs) family as an example,
previous studies have shown Co-ZIF-9 and Co-ZIF-67 with various
2
nanostructured morphology are capable of promoting the CO
2
systems involving H O as the proton source, this potential also
2
photoreduction by integrating the electron-mediating function of
cobalt species spatially confined by imidazolate motifs in the
heterogeneous crystalline scaffolds.3, 12 Particularly, a leaf-like
morphology of the two-dimensional laminated Co-ZIF-67 further
conveniently triggers the hydrogen evolution reaction (HER), which
RR.17-19
has been viewed as a major competing reaction for CO
2
While in many applications a high selectivity towards CO is
enhances its capability of CO
2
reduction by facilitating the electron
transfer between the co-catalyst and the photosensitizer
a.Soochow Institute for Energy and Materials Innovations, College of Energy,
Soochow University, Suzhou 215006, P. R. China.
Key Laboratory of Advanced Carbon Materials and Wearable Energy Technologies
2
+ 10
[
Ru(bpy)
3
] .
Nevertheless, despite of these intriguing
significant challenge remains to balance the
b.
observations,
a
of Jiangsu Province, Soochow University, Suzhou 215006, P. R. China.
photocatalytic properties with MOF stability, especially when water
is used as both solvent and proton donor.16, 26-29 To overcome this
problem, carbonization of MOFs at high temperature to construct
heteroatom-doped metal-carbon complexes with rich active sites for
*
Email: zdeng@suda.edu.cn; ypeng@suda.edu.cn.
c.
State Key Lab of Metal Matrix Composites, School of Materials Science and
Engineering, Shanghai Jiao Tong University, Shanghai, 200240, P. R. China
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
2
CO RR has been pursued as an alternate solution.
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Using carbonized MOFs as nanostructured scaffold for hosting 2.1. Chemicals
View Article Online
DOI: 10.1039/C8TA06151C
metal complexes and nanoparticles as electrocatalytically active sites
for CO RR, the faradic efficiency towards CO production and the
2
Cobalt nitrate hexahydrate (Co(NO
3
)
2
·6H
2
O) was purchased from
Sinopharm Chemical Reagent Co., Ltd.. Zinc nitrate hexahydrate
thus resulted component ratio in the electrochemically produced
syngas can be effectively tuned through the modulation of local
coordination environment.6, 30-33 For instance, by ball-milling the
iron precursors with Zn-ZIF-8 together, Jaouen et al. prepared a set
of Fe-N-C electrocatalysts containing both single-Fe-atom centers
and Fe-containing nanoparticles (NPs), and found while the former
(
(
Zn(NO
2-MeIM) was obtained from Meryer (Shanghai) Chemical
]Cl ·6H O, acetonitrile (MeCN) and
3 2 2
) ·6H O) was provided by Alfa Aesar. 2-methylimidazole
Technology Co., Ltd.. [Ru(bpy)
3
2
2
triethanolamine (TEOA) were purchased from Aladdin. All materials
were used as received without further purification. Ultrapure water
was prepared using the Sartorius, arium mini reinstwasser-system.
The CO gas (99.995%) was supplied by Suzhou Jinhong Gas Co.
2
Ltd., and the isotopic 13CO
is responsible for CO
hydrogen evolution. Similarly, through ionic exchange of Zn-ZIF-8,
Li et al. compared Ni single atoms with nanoparticles distributed in
2
reduction into CO, the latter mainly catalyze
8
2
gas (99.9%) was purchased from Sigma-
Aldrich.
2
nitrogen-doped porous carbon for electroreduction of CO and also
revealed atomically dispersed Ni active sites exhibit higher
selectivity towards CO production.34 In a later study, utilizing the
coordination chemistry in the bimetallic Zn-Co ZIF-8 the same
group fabricated atomically dispersed Co catalysts and successfully
tuned the coordination number of single Co atoms by varying the
2
.2. Synthesis of the materials
Synthesis of ZIF-8, ZIF-67, and Zn/Co-BMZIFs: ZIFs with
various Zn/Co mole ratio including 1:0, 5:1, 3:1, 1:1, 1:3, 1:5 and
0
Typically, the corresponding amount of Zn(NO
Co(NO ·6H O (8 mmol in total) were dissolved in 100 mL
methanol, and then added into 100 mL methanol containing 30
mmol of the 2-MeIM ligand under sonication for 10 min at room
temperature. The obtained solution was further aged at room
temperature (RT) for 24 h without stirring. Finally, the obtained
precipitates were centrifuged and washed with methanol 3 times and
dried in vacuum at 70 °C for overnight. Except for ZIF-8 containing
exclusive Zn and ZIF-67 containing only Co, products of bimetallic
ZIFs are denoted as BMZIFx:y, where x:y refers to the added mole
:1 were prepared based on previously published procedures.26, 34
pyrolysis temperature.32 They found the less coordinated Co-N
chemistry demonstrated significantly higher CO faradic efficiency
than both Co-N and Co NPs, with the latter two produced H in
3 2 2
) ·6H O and
2
3
)
2
2
4
2
majority. Almost at the same time, Bao et al. synthesized
coordinatively unsaturated nickel-nitrogen (Ni-N) sites doped within
porous carbon, derived from the pyrolysis of Zn/Ni bimetallic ZIF-8
with varied Zn/Ni ratios, and revealed similar trend in the
composition of produced syngas as tuned by the Ni-N
x
coordination
chemistry.35 All these electrocatalytic studies unanimously
demonstrate structure-selectivity relationship across all
a
3 2 2 3 2 2
ratio of the starting materials Zn(NO ) ·6H O vs. Co(NO ) ·6H O.
investigated catalytic systems, with the CO/H ratio in the product
2
Synthesis of C-ZIF-8, C-ZIF-67 and C-BMZIFs: 50 mg ZIF
powder with various Zn/Co mole ratio was annealed at 700 °C for 2
hours in the tube furnace with a heating rate of 1 °C /min under the
governed by the particle size and coordination chemistry of the metal
active sites. However, to our best knowledge, similar structure-
selectivity illustration hasn’t been seen in photocatalytic CO
2
N
2
(100 mL/min) atmosphere to yield rhombic dodecahedral
reduction, and further mechanistic insights into the observed
selectivity are desired.
particles. After annealing, the carbonized ZIF-8 and ZIF-67 are
denoted as C-ZIF-8 and C-ZIF-67, respectively. The carbonized
BMZIFs with different Zn/Co ratio are denoted as C-BMZIFx:y
where x:y again refers to the added mole ratio of the starting
materials, Zn(NO ·6H and Co(NO ·6H O, used for
synthesizing the parent MOF.
Based on the above considerations, herein a series of carbonized
bimetallic Zn/Co composites template by ZIFs with the uniform
rhombic dodecahedral structure and various Zn/Co ratios have been
successfully prepared by high-temperature pyrolysis for
photocatalytic applications. We find the presence of Zn in the
bimetallic frameworks enables to not only tune the overall MOF
particle size, but also effectively control the size of Co active sites
,
3
)
2
2
O
3
)
2
2
2
.3. Characterization
on the carbonized MOF scaffolds. When used as the co-catalyst for The crystal structure was characterized by powder X-ray diffraction
photocatalytic reduction of CO
under visible light irradiation with (XRD, Bruker D8 Advance with Cu Kα radiation (λ=1.54056 Å)).
]2+ (bpy = 2,2’-bipyridine) as the photosensitizer and The surface morphology and micro-structure were observed via a
Ru(bpy)
2
[
3
triethanolamine (TEOA) as the electron donor, the carbonized ZIF dual-beam electron microscope (SEM, FEI Scios) and a field-
composite with a Zn/Co ratio of 3:1 delivered a high CO yield of 1.1 emission transmission electron microscope (TEM, FEI TECNAI G2
4
-1 -1

10 µmol·g ·h . More importantly, by regulating the size of Co F20 200 kV) equipped with an energy-dispersive X-ray analyzer
active sites, the co-catalysts with different Zn/Co ratio enable to (EDX). High-angle annular dark field scanning transmission electron
effectively tune the product selectivity in the produced syngas. microscopy (HAADF-STEM, Cs-corrected TEM, FEI Titan Themis
Consequently, this study, by illustrating the structure-activity- Cubed G2 300) was used to inspect one of the pyrolyzed samples C-
selectivity relationship of the metal active sites on the carbonized BMZIF5:1 for metal dispersion in high resolution. Chemical states of
bimetallic ZIFs, sheds new light on the design of non-noble metal surface elements were probed by X-ray photoelectron spectroscopy
photocatalysts for high-efficiency CO
synthesis.36
2
RR, as well as tunable syngas (XPS, Thermo Fisher, Escalab 250Xi) using a monochromatic Al Kα
(1486.6 eV) X-ray source, with all binding energy values calibrated
by C 1s = 284.6 eV. Raman spectra were obtained on a confocal
laser Raman microscopy (Horiba Jobin Yvon, HR Evolution).
Thermal gravimetric analysis (TGA) was performed with EXSTAR
2
. Experimental section
2
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7
300 under air with a heating rate of 5 ℃/min. N
desorption isotherms were determined on Micromeritics Tristar II condition and analyzed by mass spectrosDcOoIp:y10.1w0i3t9h/Ca8nTA0o6ff1l5i1nCe
020 at 77K. Based on the adsorption branches, the Brunauer- injection method, by which 1 mL of the gas in the reactor was
adsorption- 13CO
experiment was carried out under the sameVieewxpAertricilme Oennltinael
2 2
3
Emmett-Teller (BET) method was used to calculate specific surface extracted and analyzed 10 times before and after the photocatalytic
area (SBET). Inductively coupled plasma emission spectrometer (ICP) reaction, respectively, and then the difference of average was ploted
with an OPTIMA 8000 analyzer (PerkinElmer Inc.) was used for to obtain the final 13C spectrogram. To evaluate the cyclability of the
elemental analysis of metal ions. Steady-state photoluminescence co-catalyst, a partition method was applied to compensate the
1
spectra were recorded on a FluoroMax-4 Spectroflurometer. HNMR filtration and transfer loss of the catalyst between each cycle.
Spectra were obtained on an Agilent NMR system spectrometer Specifically, in each cycle we used 2 mg of the C-BMZIF1:1 from a
(
400MHZ).
total of 20 mg for photocatalysis, and then after each 2 mg cycle, a
parallel experiment was performed for the rest of the co-catalyst
under the same condition. Afterwards, all of the co-catalysts were
mixed together again, and after washing, centrifugation and drying,
another 2 mg was taken out for the sequential run.
2
.4. Photocatalytic test
The photocatalytic reduction of CO
2
was carried out in an external
irradiation-type reactor (Pyrex glass) with a quartz window on top of
the reactor. The reactor is connected to a closed gas-circulation
system (Labsolar-6A, Beijing Perfectlight technology Co, Ltd.).
Typically, 2 mg of C-BMZIFs catalyst and 40 mg of photosensitizer
3
. Results and discussion
All ZIF samples were synthesized using metal nitrates and 2-MeIM
ligands under sonication at room temperature (Scheme 1). Fig. 1a-c
shows the particle morphology of ZIF-8, ZIF-67 and BMZIF1:1, with
all three samples exhibiting the similar rhombic dodecahedral shape.
However, their size varies significantly. While ZIF-67 has the largest
average size about 574 ± 121 nm, those of ZIF-8 and BMZIF1:1 are
[
Ru(bpy)
3
]Cl
2 2
·6H O were uniformly dispersed in a solution of
TEOA/H
2
O/MeCN (2mL/2mL/10mL) in a petri dish with a diameter
of 5 cm, and then placed into the reactor, which was cooled by
circulating water to maintain the reaction temperature at 4 oC in
order to reduce the evaporation rate of MeCN during the
photocatalytic process. After the whole system was completely
1
20 ± 15 and 100 ± 20 nm, respectively. This observation clearly
evacuated for 10 minutes to exhaust the air (no O
2
and N
2
is
shows that the addition of Zn into ZIF-67 to form bimetallic ZIFs
enables dramatically decreasing the size of MOF particles that serve
as the scaffold for hosting catalytic active sites.
Despite the difference in coordination chemistry and particle
size, PXRD analysis on ZIF-8, ZIF-67, and BMZIF1:1, however,
reveals similar diffraction patterns (Fig. 2), indicating they share the
same topologic structure. All three samples display the same type-I
detectable from the gas chromatography), 80 kPa of pure CO
2
gas
was injected into the airtight system and kept circulating. A 100 W
LED light (PLS-LED 100, 420 nm, Beijing Perfectlight technology
Co., Ltd.) was used as the light source, and the illumination distance
was kept at 7.5 cm from the sample. The gas products (CO, CH
and N ) were analyzed every 1 h by online gas chromatography
GC 2002, Shanghai Kechuang Chromatograph instruments Co.,
4 2
, H ,
O
2
2
(
2
N adsorption isotherm (Fig. S1a) with close values of specific
Ltd.) equipped with a thermal conductivity detector (TCD) and a
flame ionization detector (FID), using argon as the carrier gas. The
quantification of the CO and H was calculated by the calibration
2
2
-1
surface area (BET) being 1431, 1434 and 1382 m ·g for ZIF-8,
ZIF-67 and BMZIF1:1, respectively, which are in good agreement
with previous literature reports.16, 26, 37 TEM images of BMZIF1:1
clearly present the well-defined rhombic dodecahedral crystal
structure, with elemental mapping from the energy-dispersive X-ray
curve with a fitting equation obtained using the external standard
method. The solution products in the liquid phase were analyzed by
1
1
H nuclear magnetic resonance spectroscopy ( HNMR). The isotope
Scheme. 1 Schematic illustration of the synthesis of C-ZIFs and the role of Co active sites in photocatalysis.
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(EDX) spectroscopy showing homogeneously dispersed Zn, Co efficiency. BET analysis on the carbonized ZIFs at 70V0ie°wCAr(tiFcliegO. nSlin6e)
DOI: 10.1039/C8TA06151C
elements (Fig. S2). Thermogravimetric analysis (TGA) reveals a revealed drastically decreased specific surface areas, with the
2
-1
better thermal stability of ZIF-8, sequentially followed by the measured values of 166, 68, and 85 m ·g for C-ZIF-8, C-ZIF-67
BMZIF1:1 and ZIF-67 (Fig. S1b). As for the solvent stability, a and C-BMZIF1:1, respectively. The decrease in surface area indicates
similar trend was observed for these three samples in a mixed the pore structure of the original ZIFs is diminished after
solvent containing water, triethanolamine and methyl cyanide, which carbonization.
has been typically used in [Ru(bpy)
fact, ZIF-67 is so unstable that after overnight the purple color of the and C-BMZIF1:1 at 700°C inherit the original MOF morphology with
solution faded severely (Fig. S3). slightly shrunken surfaces. While the surface of C-ZIF-8 seems
3
]2+ mediated photocatalysis. In
SEM images (Fig. 1d-f) show the carbonized C-ZIF-8, C-ZIF-67
As a means to improve the stability while still maintaining the smooth, a large number of small nanoparticles can be visualized on
MOF scaffold, all ZIF samples were carbonized at 700 °C for 2 h the surface of both C-ZIF-67 and C-BMZIF1:1, and the particle size
under the N
2
atmosphere in a tube furnace. This annealing observed on C-ZIF-67 is significantly larger. TEM images (Fig. 1g-i,
temperature was chosen based on below considerations: First, both Fig. S7) further reveals more nanoparticles with larger size (11.8 ±
TGA curves (Fig. S1b) and Raman spectra (Fig. S4) indicate at 700 3.3 nm) in C-ZIF-67, whereas those in C-BMZIF1:1 are much less
°C all ZIF samples are sufficiently carbonized. Second, ZIFs and smaller (4.1 ± 2.1 nm). As there are no nanoparticles seen on C-
annealed at 700 °C are able to maintain their rhombic dodecahedral ZIF-8 from both SEM and TEM images, those observed on C-ZIF-
structure even after intensive sonication (Fig. 1d-1f, required for 67 and C-BMZIF1:1 must be agglomerated Co nanoparticles, which is
their homogeneous dispersion in the photocatalytic system), whereas further evidenced by elemental mapping from the SEM-EDX
those annealed at higher temperature (for example at 900 ℃) will characterization (Fig. S8). Besides, the majority of the Co
collapse and break into small debris (Fig. S5). Third, a lower nanoparticles in C-ZIF-67 are localized at the center of the MOF
annealing temperature can be used to avoid the severe Oswald dodecahedron (Fig. 1h, Fig. S7), whereas those in C-BMZIF1:1
ripening of nanoparticles, which is beneficial for the photocatalytic appear more scattered (Fig. 1i). Therefore, with the incorporation of
Fig. 1 SEM images showing the morphology of (a) ZIF-8, (b) ZIF-67, (c) BMZIF1:1 before carbonization, and (d) C-ZIF-8, (e) C-
ZIF-67, (f) C-BMZIF1:1 after carbonization. TEM images showing the Co particle size and distribution after carbonization on (g) C-
ZIF-8, (h) C-ZIF-67 and (i) C-BMZIF1:1 at 700°C.
4
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Fig. 2 PXRD of (a) BMZIFs and (b) C-BMZIFs with different Zn/Co ratios.
Zn into Co-ZIF-67 to form bimetallic ZIFs, not only the MOF size, Co peaks detectable on both C-BMZIF5:1 and C-BMZIF3:1
but also the distribution and size of Co active sites formed upon suggesting that the Co nanoparticles, if exist, are too small or less to
carbonization can be regulated. be probed by XRD. In addition, no peaks of Zn are visible across all
,
Following the lead, next we systematically varied the Zn/Co spectra, confirming their existence in the coordinated state and no
ratio in the synthesized ZIFs and carefully examined the size agglomeration of Zn nanoparticles.
evolution of MOF particles and Co active sites upon carbonization.
XPS survey spectra reveal the presence of C, N, Zn and Co
Including the ZIF-8 and ZIF-67, a total of 7 Zn/Co ratios were elements in all C-BMZIFs (Fig. S14). The high-resolution C 1s
examined. The smallest MOF particle size was observed on spectra (Fig. S15) illustrate that the dominant carbon species after
2
BMZIF3:1, with the average of 100 ± 20 nm and even smaller than pyrolysis are the sp -hybridized carbon of C=C bonds (~284.8 eV),
3
that of ZIF-8 (see Fig. S9, S10 for the trend of particle size change C=N bonds (~285.6 eV) and sp -hybridized carbon of C-N bonds
with the Zn/Co ratio). The actual Co content and Zn/Co ratio in all (~286.7 eV), and the composition of graphitic carbon increases with
BMZIFs were quantified by EDX (Table S1, S2), showing the mass the increasing Co content.38, 39 For N 1s (Fig. S16), XPS shows the
percentages of Co are actually lower than the predicted values based major species include pyridinic (~398.7 eV), pyrrolic (~399.9 eV)
on the raw material stoichiometry. Taking the BMZIF5:1 as an and graphitic (~401.1 eV) nitrogen, with an overall increase of the
example, the measured Co content of 1.16% is significantly lower graphitic nitrogen with the increasing Co content.40, 41 For all
than the theoretical value of 4.34% calculated based on the reactant carbonized samples excluding C-ZIF-67, Zn 2p3/2 and Zn 2p1/2 peaks
ratio, indicating that the actual Zn/Co ratio in the BMZIFs is can be detected and are located at ~1021.9 eV and ~1044.8 eV,
influenced by both the reactant concentration and the Ksp of the final respectively, corresponding to the coordinated state of Zn2+ (Fig.
products. Consequently, one will always need to perform elemental S17).30, 35, 42, 43
analysis to quantify the actual Co content in the BMZIFs, which is
confirmed by both EDX and ICP-AES in the current study, showing
fairly consistent values with each other (Table S1).
After carbonization, both SEM and TEM characterizations (Fig.
1
, 3 and S11) show that Co nanoparticles are formed on all C-
BMZIFs except for C-BMZIF5:1, which exhibit almost no difference
from C-ZIF-8. The lack of nanoparticles on C-BMZIF5:1 was further
confirmed by HRTEM, HAADF-STEM and the corresponding EDS
images acquired by a Cs-corrected TEM (Fig. S12), showing all
elements including Zn, Co are homogeneously dispersed in the
sample. On C-BMZIF3:1, nanoparticles with extremely fine size of
2
.3 ± 0.65 nm are visible across the whole sample with sporadic
agglomerated ones. Beyond that, both the amount and size of Co
nanoparticles monotonically increase with the decreasing Zn/Co
ratio (Fig. S13). The carbonized C-BMZIFs with various Zn/Co
ratios were further characterized by XRD to illustrate the structural
evolution. As shown in Fig. 2b, after the thermal treatment at 700
°
C, the original diffraction pattern characteristic of ZIFs (Fig. 2a)
Fig. 3 TEM images of C-BMZIFs with the Zn/Co ratio of (a)
:1, (b) 3:1, (c) 1:3, (d) 1:5. While no distinct Co particles is
visible on C-BMZIF5:1, the size of Co particles increases with
the decreasing Zn/Co ratio for the other samples.
completely disappears, accompanied with the monotonical evolution
5
°
°
of Co (111) and (200) peaks at the 2θ of 44.2 and 51.6 (PDF # 15-
806) with the decreasing of Zn/Co ratio. Surprisingly, there are no
0
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More importantly, XPS analysis of Co 2p3/2 and Co 2p1/2 reveals were illuminated by a 100 W LED light (λ = 420 nm)Vfieowr AartdiculerOatniloinne
0
DOI: 10.1039/C8TA06151C
prominent peaks of Co at 778.5 and 793.6 eV, respectively, on C- of 3 hours. Under these conditions, in all experiments CO and H
2
BMZIFs when the Zn/Co ratio is above 3:1, whereas on C-BMZIF5:1 were detected by gas chromatography (GC) as the only reduction
0
they are barely detectable (Fig. 4). The absence of the Co species products (Fig. S18a), and no other soluble products such as formate
indicates little metallic Co particles in C-BMZIF5:1, in resonance were found by the 1H nuclear magnetic resonance spectroscopy
with the above TEM and XRD analysis. Co2+ is the other major (1HNMR，Fig. S18b).12, 44 Fig. 5a and 5b present the evolution of
cobalt species existing across all Co-containing samples, and its CO and H
intensity ratio to Co decreases with the decreasing Zn/Co ratio. An C-ZIF-8, C-BMZIF1:1, and C-ZIF-67. Among these three samples,
2
，respectively, within the 3-hour illuminiation time for
0
exception, however, is the C-ZIF-67, in which a relatively low level C-BMZIF1:1 produced the highest amount of CO of 47.5 μmol,
of Co0 (compared to Co ) is observed. This is likely due to the whereas C-ZIF-67 had the highest H
2+
evolution of 56.4 μmol. C-
2
much larger size of C-ZIF-67, and the fact that the majority of Co ZIF-8 barely exhibits any catalytic reactivity, and shows no
nanoparticles are localized at the center of the polyhedron, as XPS is distinguishable difference in both CO and H production from the
2
a surface-sensitive technique. Previous studies have also shown that control experiement without any co-catalysts (Fig. S19). This
the agglomeration of metal nanoparticles can be effectively inhibited confirms that Co sites in both C-BMZIFs and C-ZIF-67 are soly
by the largely dispersed Zn in bimetallic ZIFs, even resulting in responsible for their catalytic behaviors.
atomic metal sites with various nitrogen coordination.32 This could
Extending the photoreaction time beyond 3 hours (Fig. 5c), we
be the case of C-BMZIF5:1 in the current study, as HR-TEM, start to see the rate of CO evolution slows down, possibly due to the
HAADF-STEM, XRD, and XPS all failed to detect any metallic deactivation of the photosensitizer. It is well known that in such
cobalt particles. However, to further prove this, more quantitative hybrid photocatalytic systems the deactivation of photoabsorbers
characterizations such as Extended X-ray absorption fine structure poses a common issue,3, 45 but the deactivation of the co-catalyst also
(EXAFS) are needed. Nevertheless, our investigations cannot be ruled out. Therefore, in order to confirm whether the slow-
unambiguously demonstrate that varying the Zn/Co ratio in down of CO evolution is caused by the deactivation of the co-
bimetallic ZIFs allows regulating the size and distribution of Co catalyst, we peformed two stability tests. In the first relay test, at the
active sites on the carbonized MOF scaffold.
The catalytic potency of all carbonized ZIFs was evaluated in a was replenished into the photocatalytic system and the reaction was
hybrid photocatalytic system, using [Ru(bpy)
]2+ as the found resumed (Fig. 5c). In the second cycling test, we recycled the
photosensitizer and TEOA as the electron donor. In a typical C-BMZIF1:1 co-catalyst by filtration (see the photocatalytic test for
experiment, a 14 mL solution of MeCN/H O/TEOA (10:2:2, v/v/v) experimental details) and examined its cycling stability (Fig. 5d). A
containing 2 mg of the co-catalyst and 40 mg of [Ru(bpy) ]Cl ·6H total of 3 consecutive cycles were carried out using the same co-
end of the 6th hour, an equal amount of photosensitizer [Ru(bpy) ]2+
3
3
2
3
2
2
O
Fig. 4 XPS Co 2p spectra of (a) C-BMZIF5:1, (b) C-BMZIF3:1, (c) C-BMZIF1:1, (d) C-BMZIF1:3, (e) C-BMZIF1:5, (f) C-ZIF-67. The
0
evolution of Co species is distinguishable on all samples with the Zn/Co ratio above 5:1.
6
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catalyst, showing no sign of performance decay. What’s more, SEM V (vs. NHE) based on the Mott-Schottky analysis (Fig. S21b), lower
View Article Online
DOI: 10.10 23+ 9/C8TA06151C
3
+
and PXRD characterizations were performed on the recycled C- than the redox potential of E([Ru(bpy)
3
] /[Ru(bpy)
3
] * ) = -0.87 V
BMZIF1:1 after photocatalysis to verify their structural integrity (Fig. (vs. NHE) and higher than that of E(CO
2
/CO) = -0.53 V (vs.
S20), and both analysis reveal no structural change of the co-catalyst NHE).46-48 Both the PL and Mott-Schottky analysis indicate C-
throughout all conducted experimental cycles. These results clearly BMZIF1:1 are capable of mediating the electron transfer from the
3 3 2
] /[Ru(bpy) /CO. Third, when
]2+* to CO
3
+
demonstrate that C-BMZIFs as the co-catalyst in our hybrid redox couple [Ru(bpy)
photocatalytic system possesses excellent stability and durability, the irradiation was cut from the experiments, there was also no
whereas the deactivation of the photosensitizer is the limiting factor detectable CO and confirming the reaction is indeed
for extended CO RR. photocatalyzed. Fouth, when there was no supply of CO with all
To justify the role of the co-catalyst in our hybrid photocatalytic other conditions maintained the same, again no CO was detected
system, various control experiments were carried out under different except for H (18.4 μmol), indicating that the above produced CO is
conditions (Fig. S19). First, without using the co-catalyst, there was exclusively from CO experiments were
. Moreover, isotopic 13CO
only 6.1 μmol CO and 4.6 μmol H produced in 3 hours, whereas the conducted under identical conditions with the reduction products
aforementioned complete photocatalytic system containing 2 mg C- analyzed by GC and mass spectra. A strong peak at m/z = 29 (Fig.
BMZIF1:1 and 40 mg [Ru(bpy)
]2+ produced 47.5 μmol CO and 47.4 S22), assignable to 13CO, is present in the mass spectra, proofing the
μmol H . Second, when there was no photosensitizer used in the carbon source of CO does indeed come from the CO , rather than
photocatalytic system, neither CO nor H was detected, indicating C- other organic reagents such as TEOA. Lastly, the supernatant after
BMZIF1:1 alone is not sufficient to trigger the catalytic reaction, and photocatalysis was collected by filtration for ICP analysis of possible
the electrons for CO
reduction come from photoelectrons generated Co2+ leaching, showing no Co species within the detection limit.
by the photosensitizer. This point of view is further confirmed by Moreover, photocatlytic CO reduction was carried out using the
photoluminescence (PL) characterizations (Fig. S21a), in which the filtered supernatant with fresh [Ru(bpy)
]2+ and TEOA, exhibiting
characteristic emission peak of the photosensitizer at about 620 nm no distinct CO and H evolution after 3 h illumination (Fig. S19).
2
H ,
2
2
2
2
2
2
3
2
2
2
2
2
3
2
monotonically decreases with the addition of C-BMZIF1:1. In These experiements confirm that the photocatalytic reaction in the
addition, the flat band potential of C-BMZIF1:1 is measured ca. -0.76 current study is a heterogeneous catalytic process.
2
Fig. 5 Photocatalytic performance of C-ZIF-8, C-ZIF-67 and C-BMZIF1:1. (a) CO and (b) H evolution vs illumination time in the
MeCN/H
2
O/TEOA solution (14 mL 10:2:2 v/v/v), containing 40 mg [Ru(bpy)
3
]Cl
2
·6H
2
O and 2 mg co-catalysts; (c) the relay stability
and (d) the cycling stability test for C-BMZIF1:1
.
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View Article Online
DOI: 10.1039/C8TA06151C
2 2
Fig. 6 Histograms for comparing the CO and H evolution rate and turnover frequencies. (a) The total evolution of CO and H within the
3
h photocatalytic reaction for C-BMZIFs with various Zn/Co ratios. (b) the corresponding TOFs based on the mass percentage of Co
quantified by EDX. (c) Proposed selectivity cause of the photocatalytic CO reduction to syngas by C-BMZIFs.
2
To further interogate the correlation of the size of Co active sites which is likely due to the mono-dispersion of Co active sites within
with their catalytic activity and product selectivity, systematic the carbon matrix. On the other hand, C-BMZIF3:1 gives the highest
photocatalytic experiments of CO
RR were carried out using C- yield of CO, 1.1  10 µmol g h-1, as a result of the optimal balance
4
-1
2
BMZIFs with different ratio of Zn/Co as the co-catalysts. As shown between the quantity of active sites and the activity of each
in Fig. 6a, without Co the C-ZIF-8 exhibits no activity and both CO individual site. In comparison to previous reports of photocatalytic
2 2 3
evolution are similar to those acquired without adding any CO reduction using ZIF-67 as the co-catalyst and [Ru(bpy)
]2+ as
and H
co-catalysts (Fig. S19). Then, with the increasing of Co ratio in the the photosensitizer, the catalytic activities revealed in this study for
bimetalic ZIFs, the H evolution increases monotonically. The CO carbonized BMZIFs are comparable to those obtained for ZIF-67,
2
evolution, however, does not correlate well with the Co content and ranging from 3.89  10 µmol g h-1 to 5.9  10 µmol g h .
a maximal value is observed for C-BMZIF3:1. Based on the statistics However, when taking into account the mass percentage of Co in the
of Co particle size distribution (Fig. S13), it is clear that the co-catalyst and the stability of carbonized materials, C-BMZIFs
3
-1
4
-1 -1 3, 10
evolution rate of H
CO is more favored on Co sites with small particle size. system of this study, the catalytic activity and product selectivity are
Accordingly, the CO/H ratio in the produced syngas exhibits an a comprehensive outcome of various contributing factors. First, the
2
is higher on larger Co particles, whereas that of exhibit some intrinsic advantages. In the hybrid photocatalytic
2
overall trend of descending with the decreasing Zn/Co ratio in the size of the Co active sites consitutes the major influential factor on
bimetallic ZIFs. In addition, a rough comparison can be made on the the product selectivity by promoting the CO evolution rate on
lower bound of the turnover frequency (TOF) for both CO
and H evolution, assuming all Co elements detected by EDX in the the overall gas production, this enhancement effect is constrained by
co-catalyst participate to the conversion reactions (Table S3). Fig. 6b the agglomeration of Co particles when less Zn is contained in the
summarizes the thus obatained TOFs on both CO and H evolution bimetallic ZIFs. As a result, although the Co cotent of C-ZIF-67 is
for C-BMZIFs with various Zn/Co ratios. One one hand, C- over 20 times higher than that of C-BMZIF5:1, the total product yield
2
reduction smaller Co particles. Second, while higher Co content is in favor of
2
2
2 2
BMZIF5:1 demonstrates the highest activities for both H and CO (including both H and CO) of the former is only 1.5 times of the
evolution, with a high TOF of 3.4  10 s-1 for HER and 6.5  10-3 s- latter. Therefore, there exists a optimal Zn/Co ratio that balance both
-3
1
for CO
2
RR, respectively. Notably, the TOF of CO
2
RR for C- catalytic activity and production yield. Third, the particle size of
BMZIF5:1 is 23-fold higher than that for C-ZIF-67 (0.28  10-3 s ), MOFs that serve as the scalfold for hosting the Co active sites also
-1
8
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plays an important role by affecting their spatial distribution, as well efficient CO
as the specific surface area for reaction kinetics. Consequently, the syngas production with tunable CO/H
highest product yield with high CO/H ratio was obtained by C-
2
RR and offers an alternative solution for photocatalytic
View Article Online
ratio. DOI: 10.1039/C8TA06151C
2
2
BMZIF3:1, in which both of the size and exposed quantity of Co
active sites are balanced to an optimal level.
Conflicts of interest
Further regarding the photocatalytic process and product There are no conflicts to declare.
selectivity, we were able to propose a reaction mechanism based on
the above observations and discussions. As illustrated in Scheme 1,
]2+ is excited to
Acknowledgements
upon illumination the photosensitizer [Ru(bpy)
Ru(bpy)
]2+*, which is a highly potent electron donor. Then, at This work was supported by the 1000 Young Talents Program of
elevated energy levels, the photoexcited electron of [Ru(bpy)
]2+* China, Natural Science Foundation of China (No. 21701118),
3
[
3
3
could be readily delocalized from its π* orbital and further Natural Science Foundation of Jiangsu Province (No. K20161209
transferred to the Co active sites.47, 49 Simultaneously, a hole will be and No. BK20160323) and the Key Technology Initiative of Suzhou
left in the HOMO orbital of the sensitizer, accepting an electron Municipal Science and Technology Bureau (SYG201748). We also
from the readily available TEOA with the sensitizer acting as an extend our sincere appreciation to the support by Suzhou Key
oxidant. The highly reductive electrons transferred to the Co active Laboratory for Advanced Carbon Materials and Wearable Energy
sites will further interact with the surface-absorbed proton and CO
species to produce H and CO through a two-electron transfer
process. Moreover, the observed size-dependent product selectivity
can be rationalized by the following explanation and Fig. 6c. Due to References
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Journal of Materials Chemistry A
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DOI: 10.1039/C8TA06151C
Activity and Selectivity Regulation through the Size of Cobalt Active
Sites in Photocatalytic CO Reduction
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